The invention relates to a process for the extraction of cellulose, and bleached cellulose and chemical cellulose, which can be obtained in accordance with this process.
Conventional processes for the extraction of cellulose, such as the sulfite and sulfate processes, lead to discharges which contain sulphur, in the combustion of which waste gases which contain SO.sub.2 arise. The high residual lignin contents of the celluloses, of 4% to 5% by weight, require large quantities of bleaching chemicals, which lead to chlorinated organic compounds in the waste waters. It is disadvantageous in this process, furthermore, that, because of the re-extraction of the chemicals through the combustion of the discharges, plants with a minimum capacity of 1,000 thousand tons per day of cellulose are necessary, There is described, in U.S. Pat. No. 3,553,076, a wood pulp with aqueous acetic acid under pressure at 150.degree. C. to 205.degree. C., in which celluloses with residual lignin contents of 2% to 3% by weight (corresponding to cap figures of 12 to 20) are obtained. According to DE-A-34 35 132, the wood can be pulped even at normal pressure, if catalytic quantities of hydrochloric acid are added to the acetic acid (the acetosolve process). The residual lignin contents of the cellulose do not, in any event, decline, and chloride ions perform, in the presence of acetic acid, in a strongly corrosive manner.
Other mineral acids, such as sulphuric acid, phosphoric acid, perchloric acid, MgCl.sub.2, or nitric acid have been investigated as catalysts for use during wood pulping with acetic acid, but have, however, yielded celluloses, without exception, which have higher residual lignin contents and lead to problems in the recovery of the mineral acids.
Formic acid has also been proposed as a means for wood pulping. Thus, in a two-state process, chopped scraps are treated in the first stage with formic or acetic acid and, in the second stage, hydrogen peroxide is added and heated up to 70.degree. C. to 100.degree. C. The quantities of hydrogen peroxide which are necessary for this, however, are too high in relation to an economic process management (Poppius et al., Paper and Timer, 73 (2), pages 154-158 [1991]).
It is an object of the present invention to provide a process for the extraction of cellulose, by means of which celluloses with distinctly lower residual lignin are obtained.
This object is solved by means of a process in which lignocelluloses with aqueous acetic acid are heated under pressure and the addition of formic acid.
Wood or annual plants can be used as the initial celluloses. The pulping temperature preferably lies between 130.degree. C. and 190.degree. C. The concentration of the acetic acid in the pulping medium is, preferably, between 50% and 95% by weight; that of the formic acid below 40% by weight; and that of the water below 50% by weight. The weight ratio of the lignocellulose to the pulping solution preferably amounts to 1:1 to 1:12.
In accordance with another form or implementation, the process can also be used for the extraction of lignin and hemicelluloses from lignocelluloses. The process management can take place either continuously and discontinuously whereby, in the event of a continuous process management, the crushed lignocellulose is fed into a pressure cooker, in which it is extracted from the pulping solution in the counter-current, and continuously leaves the cooker at the other side in extracted form. By this means, for example, 2 to 20 pulping vessels can be connected in series, one after the other.
In accordance with additional preferred forms of implementation, the shredding of the cellulose and the washing process of the cellulose is included in the process in accordance with the invention. The lignocelluloses can, in order to remove the contents, be pre-extracted with a solvent, and acetic anhydride and bleaching agent can be added to the pulp solution. In accordance with one additional preferred form of implementation, the lignocelluloses are impregnated, before being fed into the pulping vessel, with formic acid, acetic acid, acetic acid anhydride, or the vapors of the same. The impregnation can also be carried out with a solvent or the vapors of the same, which forms an azeotrope with water.
The lignins and hemicelluloses with high degrees of purity which are likewise accumulating can be used, for example, for the production of glue.
The process in accordance with the invention has the advantage, relative to the conventional process for the extraction of cellulose, that it does not use any inorganic pulping chemicals, so that no exhaust gases which contain SO.sub.2, or waste waters which contain heavy metals, are thereby brought about. Formic and acetic acids are recovered by means of distillation, so that the lignins and the hemicelluloses do not need to be subjected to combustion in order to recover the chemicals. One additional advantage consists of the fact that the pulping temperature is approximately 10.degree. C. lower than it is in the conventional process, as the result of which the costs for energy are considerably reduced.